1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a method of manufacturing the same, and more particularly, to a laser applied to a light source for an optical data processing and for a data recording.
2. Description of the Related Art
In the technical field of the optical communication and the optical recording, an interest to a surface emitting semiconductor laser (Vertical Cavity Surface Emitting Laser Diode, hereinafter referred to as VCSEL) has been increased in these years.
VCSEL has technical advantages that a threshold current is small, an optical spot of a circular shape can be easily obtained, and an evaluation at a wafer condition and two dimensional array of the light source can be achieved. “Small volume of the active region” causes the low threshold current while there is a trade off that a device resistance is dozens of to hundreds of Ohms, which is higher than that of an edge emitting semiconductor laser, and that obtaining a high optical output (not less than 10 mW) by itself is difficult.
The optical communication using an optical fiber has applied to a data transfer for a relatively middle or long distance (from several to dozens of kirometers). Typically, a combination of a single mode optical fiber and a Distributed-Feedback type (hereinafter referred to as DFB) laser is used. The DFB is oscillated at a wavelength range such 1.31 micron meter band or 1.55 micron meter band, which is a small distribution or loss in the optical fiber. They are used for communication associations who are referred to as a main line system and are main users and the production volume is not large compared with consumer products. It makes the price expensive. Furthermore, the system in itself has many problems of blocking a low price since it needs to control a device temperature and to take time for adjusting optical axes of the optical fiber and the laser.
These days, ADSL and CATV become widely used for families, and a high capacity data transfer, which is ten times to hundred times compared with the past, has been achieved. Increasing of the Internet user in future will be expected. Additionally, a demand for a high speed and a high capacity of data transfer will be enhanced and it is promising that many families will utilize the optical fiber in some day.
However, the combination of the single mode type of optical fiber and DFB laser for the middle/long distance, for example a few meters to dozens of meters of data transfer between home and an telegraph pole, is diseconomy. For the short distance communication (several to hundreds of meter), using a low cost optical fiber such as a multi-mode type of silica fiber or a plastic optical fiber (POF) is economical. Thus, the light source used for the multi-mode type of the optical fiber is required to be an affordable and not to need a particular an optical system and a driving system. VCSEL would be an option since it can satisfy with all above requirements.
VCSEL available at the present market is structurally classified into an ion-planting type and a selective-oxidation type. If the high speed response is required in the optical communication in future, latter type would be mainstream. VCSEL of this type has a strong effectiveness for optical confinement and provides excellent electrical-optical characteristics of a high efficiency and a low threshold current because a part of a semiconductor multiple layer reflecting mirror adjacent to an active region is selectively oxidized to form a refractive-index wave guide therein. Additionally a modulation band (3 dB down cut-off frequency) reaches at a few giga-herz even though a low bias current is a few milliampere, the response characteristics is excellent in the high speed modulation.
The leading role of the local area network (LAN) of an indoor network is Ethernet and its data communication rate starts at dozens of mega bit per second (Mbps) and has made a progress at hundreds of mega bit per second. Recently the rate of 1 giga bit per second (Gbps) has appeared and rate would get to hundreds Gbps in near future. Until 1 Gbps, an electric wiring using a twist pair cable can be applied, but it is considered that an optical wiring would take over it more than 1 Gbps because of a limit in the view of noise tolerance.
There is an aggressive trend to adopt VCSEL in the light source used for the optical wiring in the Ethernet of 10 Gbps and a development has been advanced. As explained above, there is no problem of the modulation at several GHz, but any measure is necessary to improve the response characteristics over 10 GHz.
A theoretical review of the modulation band for the semiconductor laser is disclosed in “Semiconductor Laser”, Kenichi Iga, Ohmsha, 1990”. 3 dB cut-off frequency (f3 dB), which is indication of the modulation band, is represented as formula (1)
                              f                      3            ⁢            dB                          =                  1                      2            ⁢            π            ⁢                                                  ⁢            CR                                              (        1        )            
Namely, the response characteristics depend on CR time constant. The above paper also explains that a relaxation-oscillation-frequency (fr) which is a target of an improvement of the modulation band is conducted by formula (2) with a rate equation.
                              f          r                =                              1                          2              ⁢              π                                ⁢                                                    ξ                ⁢                                                                  ⁢                                  G                  ′                                ⁢                                  P                  out                                                                              η                  d                                ⁢                h                ⁢                                                                  ⁢                                  ω                  o                                ⁢                                  V                  m                                                                                        (        2        )            
Where ε is optical confining coefficient, G′ is a derivation gain coefficient, Pout is an optical output, ηd is an external derivation quantum efficiency, Vm is a mode volume of a resonator, h is a plank constant. Namely, it is understood that the relaxation-oscillation-frequency is increased proportion to a square root of the optical output.
As explained the above, it is understood that there are some points to be considered, such as a low capacitance, a low resistance and a high output power, for improving the modulation response of not only VCSEL but also the semiconductor laser
From these standpoints, U.S. Pat. No. 5,343,487 discloses a preferable embodiment structure for improving the modulation response of VCSEL. This structure is known as an intra-cavity-contact structure. The lower and upper electrodes of different conductivity type are formed so as to sandwich an active region, their contacting positions are placed inside of the lower and upper multiple layer reflecting mirrors forming a resonator, therefore, this structure is referred to as the intra-cavity-contact structure.
In a prior art, VCSEL usually forms the electrode outside (substrate surface and its backside) of the resonator and an injection current passes through a multiple layer reflecting mirror (almost semiconductor), as the result an energy barrier generated in the multiple layer reflecting mirror causes an increase of series resistance and an operation voltage become high. Also, since the lower and upper electrodes are opposed in a plane parallel to the substrate, this forms a parallel plate capacitor and a parasitic capacitance is produced.
On the other hand, in the intra-cavity-contact structure as shown in U.S. Pat. No. 5,343,487, the current is passed through a part of the multiple layer reflecting mirror or is injected into the active region without passing, thus the increase of series resistance resulting from the multiple layer reflecting mirror can be inhibited and the parasitic capacitance resulting from the electrode can be eliminated.
Japanese Laid-Open Patent Application No. 2001-135890 discloses a structure for reducing a parasitic capacitance of a laser device for improving the modulation response of VCSEL. The laser device includes the upper and lower reflecting mirrors for making a resonator on a substrate, the active region sandwiched by the reflecting mirrors, the upper and lower contact layers with the high conductivity which are formed at the opposite side of the substrate from the view of the active region, the upper and lower electrodes formed on the upper and lower contact layers respectively and placed both sides from the view of the upper reflecting mirror in-plane direction, and the current guide structure having the opening for injecting the current to the relative narrow channel of the active region. The part of the lower contact layer which extends at below of the upper electrode has the relatively high resistance. In the preferred embodiment, the high resistance part does not extend inside edge of the upper contact layer which has the shape of “U” from the above view. That is, the resistance portion is patterned so that the high conductive region or the finger portion can be left along the inside edge of the electrode of “U” shape. According to this, the parasitic capacitance can be reduced considerably and the remarkable increase of the device resistance can be prohibited. Furthermore, the structure made of the upper and lower contact layers is asymmetric with respect to the axis of the resonator, the side where the upper electrode is formed is wider while the opposite side is narrower, which makes the parasitic capacitance reduced more effectively.
Japanese Laid-Open Patent Application No. 11-145560 relates to the surface emitting semiconductor laser using the selective oxidation of the AlAs layer or AlGaAs layer of the current confining layer. It discloses the technique for introducing the semiconductor multiple layer mirror served as the etching stop layer. It includes the compound semiconductor material having the phosphorous group or having at least one layer containing such material between the upper reflecting mirror 10 and the current confining layer 7, for stopping an etching accurately directly above the current confining layer 7. By using the etching stop layer 12, the etching can be stopped directly above the current confining layer correctly and the yield is also improved.
However, each structure disclosed in U.S. Pat. No. 5,343,487 and Japanese Laid-Open Patent Application No. 2001-135890 needs an advanced semiconductor technique in the manufacturing process, it means a reproducibility is poor and the device structure is not practical. Especially, the method by latter assumes the structure having a gain-wave-guide that oxygen, fluorine, or helium is ion-implanted, and this structure may cause a low emission efficiency and characteristics fluctuations due to the difficulties in controlling a depth of ion-implantation. Furthermore, the structure disclosed by Japanese Laid-Open Patent Application No. 11-145560 is not the intra-cavity-contact structure, the resistance from each electrode to the upper and lower reflecting mirrors can not be suppressed. As explained the above, the conventional surface emitting semiconductor laser does not bring out the excellent features of a high efficiency and a low threshold current of the selective oxidation VCSEL, and an improved new structure has not been created yet, namely it is necessary that a device of a low capacitance, a low resistance, a high output power and a modulation response over 10 GHz can be manufactured easily in a good reproducibility.